The voltage required to induce a gas discharge is generally higher than that required for maintaining such a discharge. Generally when a high frequency voltage is applied to parallel metallic electrodes, the resulting discharge occurs only through a quite small part of the volume of the gaseous medium. After the start of the discharge the voltage drops to a lower value, and the discharge will not spread to a greater part of the volume. Hitherto, in order to obtain a gas discharge through a large part of the volume of the gas between electrodes, there are used ballast resistors, radio frequency excitation, magnetic field excitation, reactive coil excitation and the like. In Lasers and Optronics, May 1988, 20-21 there is described a laser operated by DC voltage magnetic field and ballast resistors, which ballast resistors ensure that the electrodes will be at a high voltage in case that no ignition occurs. After such ignition, the voltage difference between the power supply and the electrode voltage drops on the resistor.
Other lasers are operated by the use of a radio frequency voltage, resulting in a discharge over the full length. High power carbon dioxide lasers are excited by a radio frequency voltage applied to parallel elongated metallic electrodes. In such systems, when the frequency is lower than about 10 MHz, the discharge tends to arc and to take place only along a small section of the length of the electrodes. Generally power supply means required are rather expensive, resulting in a high cost of the entire system.